Touch sensitive display device

ABSTRACT

A touch sensor is disclosed comprising a display device having a substrate on which substrate at least one display electrode is disposed for the display of a shape on the display device. An interface is coupled to the at least one display electrode for receiving display data to the display device. Moreover is a measuring circuit coupled to the at least one display electrode. Switching means are provided for connecting the interface to the at least one display electrode when the switching means is in a first state of operation and connecting the measuring circuit to the at least one display electrode when the switching means is in a second state of operation.

This application is a United States National Phase Entry ofInternational Application No. PCT/SE2004/001447, filed Oct. 12, 2004,which claims priority from Swedish Patent Application No. 0302711-7,filed Oct. 13, 2003, and the benefit of U.S. Provisional Application No.60/516,314, filed Nov. 3, 2003, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Electrical equipment from various fields of application, e.g. mobiletelephones, personal digital assistants (PDA), and industrial controlequipment often use a display device of some sort for providing theoperator of the device with information. In simpler applications thedisplay device is a one-way communication link, i.e. the display is usedfor providing information to the operator but not to receive informationthe other way back. In order to achieve interaction with the operator,push buttons or keyboards are normally used. If the electrical equipmentis small sized, for example as with a PDA, normally no room is left onthe device for a keyboard, wherein the manufacturer of the PDA mustprovide other means for enabling input of data into the device.

As is well known in the art the input means may be in form of a touchsensitive display making it possible to enter data without the need fora separate keyboard. Many different techniques for providing touchsensitive devices have been presented and the most common solution todayis to use a separate transparent touch sensitive layer which is placedon top of the display. The touch sensitive layer is normally in form oftwo flexible superimposed plastic sheets that are separated by a smalldistance by means of insulating spacers. On the surfaces of the sheetsfacing towards each other, a matrix-like pattern of electricalconductors are arranged which pattern establishes an electric contactbetween the sheets at the location where the touch sensitive layer isdepressed. A control unit scanning the matrix-like pattern on theplastic sheets may then detect the electric contact between the sheetsand determine the coordinates for the depression on the display.

Even though the separate touch sensitive layer makes it possible toenter data into the device without the need for a keyboard, it is not anefficient way of realising a touch sensitive display since thetransparency of the touch sensitive layer is not absolute hence makingit difficult to view the information presented on the display undercertain circumstances. The unsatisfactory transparency of the touchsensitive layer is even more noticeable when the display device isprovided with back lighter or front lighter technology for making itpossible to view the information on the display under poor litconditions.

Another approach for providing a touch sensitive display is to provide adisplay with a sensor arranged under the display rather than on top ofthe display. The sensor then has to detect a touch on the display not bymeans detecting an electric contact between conductors as with thesolution disclosed above, but by using capacitive or reflectiveproperties of the display. In the former case, a capacitive couplingthrough the display to the finger touching the display makes it possibleto detect a touch on the display as well as determine the position ofthe touch. In the latter case light or sound utilizing changes in thereflective properties of the display at the point of contact may be usedfor detecting a touch on the surface of the display.

Attempts have been made to provide touch sensitivity for displayswithout the use of separate sensors arranged on top or below the displaysurface. An approach is to use the display electrodes forming the pixelsor the segments of the characters on the display for sensing the touch.

U.S. Pat. No. 5,043,710 discloses a touch sensor comprising a liquidcrystal display (LCD), wherein a touch on the display is sensed bydetecting changes in the dielectric properties of the display. Amechanical force applied to the LCD perpendicular to a flexible glasssubstrate over one of the display electrodes gives rise to a temporarydisorganisation of the molecules in the liquid crystal thereby changingthe dielectric constant of the liquid crystal under the displayelectrode. Each display electrode of the LCD is connected to anintegrator, wherein a change of the dielectric constant of the liquidcrystal when the segments of the LCD are in an excited state gives riseto an electric pulse indicating a touch on the LCD. However, thesolution according to U.S. Pat. No. 5,043,710 becomes complex due to thelarge amount of integrators needed for sensing a touch. Moreover, forsensing a touch the front glass plate needs to be flexible making thedisplay less durable. In addition to this, the working life of thedisplay is also decreased due to the repeated compressions of the liquidcrystal in the display.

U.S. Pat. No. 4,224,615 discloses a LCD with a flexible front plate,which LCD may be used as a device for receiving data from a humanoperator. An operator of a device comprising the touch sensitive displaytouches the flexible front plate of the display, wherein the front platedeflects towards the back substrate thereby increasing the capacitancebetween the display electrodes residing in the area being depressed. Thecapacitance measured between the front and back display segment iscompared with the capacitance of a reference cell, wherein it ispossible to detect a touch even if the affected display segments areactuated, i.e. presenting a shape on the display. As with U.S. Pat. No.5,043,710 the invention according to U.S. Pat. No. 4,224,615 uses thechange in dielectric constant of the liquid crystal being compressed forsensing a touch. The same problems with robustness and life expectancyas with the invention according to U.S. Pat. No. 5,043,710 exist in thesolution according to U.S. Pat. No. 4,224,615.

US 2001/0020578 discloses a LCD with touch sensitivity, wherein thesensor arrangement is placed below a surface of the display. The sensorsare preferably placed below the display in the regions of the displaywhere no display segments are arranged. Alternatively, the displaysegments of the display may be used as sensors provided that the frontand back segment are short-circuited. When the display electrodes act astouch sensors, no information may be presented on the screen due to theshort-circuiting of the display electrodes. A microprocessor istherefore coupled to the display segments for alternating betweenpresentation of information on the display and touch sensitivity.

U.S. Pat. No. 4,910,504 discloses a touch controlled display device,wherein a touch on the display is sensed by measuring the capacitancebetween different display electrodes on the front substrate. The fontsubstrate may then be rigid protecting the display from deformation. Thedetector measuring the capacitance between the electrodes is coupled tothe feeding pins of the display. A common counter-electrode is arrangedon the back substrate in a manner known per se. As will be disclosedbelow, the counter-electrode will act as a short-circuit between theelectrodes on the front substrate thereby deteriorating the accuracy ofthe touch sensitive display in regard of where on the screen the touchis made. Moreover, numerous stray-capacitances in the needed drivecircuitry for the display will interfere with the capacitance measuringcircuitry making it hard to determine where and if a touch is made.

DE 19802479 discloses a touch-sensitive display for use in e.g.elevators. The front surface of display element is provided with anelectrically conducting layer which is so thin that the display elementis visible through the conducting layer. An evaluation circuit isconnected to the conducting layer in order to detect a touch on thedisplay. However, by arranging a conductive layer in front of thedisplay element, the visibility of the display element is deteriorated.Moreover, the conductive layer will be exposed to wear from users of thedisplay, which implies that the endurance of the display will beinsufficient for many applications.

For manufacturers of display driver circuits it is of most importancethat the circuitry used for detecting a touch on the screen is notaffecting the behaviour or the life-expectancy of the driver circuitry.Hence a touch sensitive display which behaves like a “normal” displayfrom a drivers point of view is hence wished for.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the above describedproblems of the known technologies in regards to providing a touchsensor which is durable and provides a reliable detection of touch onthe display. The present invention is based on the understanding that adisplay is associated with specific physical characteristics whichinfluence the reliability of the detection of a touch on the display.

Particular advantages of the present invention are reliability of thedetection of a touch on the screen, improved robustness of the touchsensor, and the improved matching towards available display drivercircuits.

A particular feature of the present invention relates to the provisionof a touch sensor with a basic configuration making it possible toreliably detect a touch on the display without deforming the display orrequiring specially adapted display driver circuitry. The designer of asystem comprising a touch sensor according to the present invention maythen freely choose driver circuits thereby lowering the overall cost ofthe system.

The above objects, advantages and features together with numerous otherobjects, advantages and features, which will become evident from thedetailed description below, are obtained according to a first aspect ofthe present invention by a touch sensor comprising:

a display device having a substrate on which substrate at least onedisplay electrode is disposed for the display of a shape on the displaydevice;

an interface coupled to the at least one display electrode for receivingdisplay data to the display device;

a measuring circuit coupled to the at least one display electrode;

switching means for connecting the interface to the at least one displayelectrode when the switching means is in a first state of operation andconnecting the measuring circuit to the at least one display electrodewhen the switching means is in a second state of operation.

A touch sensor according to the present invention will hence be able toreliably detect a touch on the display by means of a capacitancemeasuring circuit even though large stray capacitances are present inthe display.

A touch sensor according to the present invention may comprise ameasuring circuit which is a capacitance measuring circuit.

A touch sensor according to the present invention may comprise ameasuring circuit which is a resistance measuring circuit.

According to the present invention the measuring circuit may comprise asignal generator coupled to the at least one of the display electrodesfor providing a predetermined test signal to the display electrode, anda signal evaluating circuit coupled to the at least one displayelectrode for receiving the test signal from the signal generator.

According to the present invention the signal evaluation circuitry maybe adapted to detect a deviation in the test signal when the switchingmeans is in the second state of operation.

According to the present invention the signal generator may be adaptedto apply the test signal to the segments on a back substrate or to thesegments on a front substrate of the display device.

According to the present invention the segments on the substrate whichis not connected to the signal generator may be left in a high-ohmicstate.

The present invention also relates to a method for detecting a touch ona display having a substrate, on which substrate at least one displayelectrode is disposed for the display of a shape on the display device,wherein said display electrode is coupled to an interface for receivingdisplay data to the display device, the method comprising the steps of:

disconnecting the at least one display electrode from the interface;

connecting said display electrode to a measuring circuit; and

detecting a change in an electrical property of the display electrodedue to an electrical coupling towards an object touching the displaydevice in the vicinity of the display electrode.

The method according to the present invention may comprise the steps ofapplying a predetermined test signal to the display electrode anddetecting a deviation in the test signal due to an electrical couplingtowards an object touching the display device in the vicinity of thedisplay electrode.

The method according to the present invention may detect a capacitivecoupling towards an object touching the display device in the vicinityof the display electrode.

The method according to the present invention may detect a galvaniccoupling towards an object touching the display device in the vicinityof the display electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionin conjunction with the appended drawings.

FIG. 1 a illustrates the structure of a display known per se;

FIG. 1 b illustrates the disposition of some of the stray capacitancesassociated with a display known per se;

FIG. 2 is a schematic diagram of a touch sensor according to a preferredembodiment of the present invention;

FIG. 3 is a more detailed illustration of the function of the touchsensor according to a first embodiment of the present invention; and

FIG. 4 is a more detailed illustration of the function of the touchsensor according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The most common display used today is the liquid crystal display (LCD)whose design and operation is well-known to the skilled person. Variantsof the LCD display, e.g. Thin Film Transistor Displays (TFT) as well asother display techniques, such as Plasma Display Panels (PDP), VacuumFluorescent Displays (VFD), Ferroelectric Liquid Crystal displays (FLC),Surface-stabilized cholesteric texture-type (SSCT) displays, organicLight-Emitting Diode (OLED) displays, and Liquid Crystal on Silicon(LCOS) displays are commonly used depending on the specific field ofapplication. For the sake of simplicity the following text will disclosea touch sensitive display in form of a LCD, wherein a change incapacitance in the display is detected. The present invention is,however, not limited to such a display, but may be implemented on adisplay of any kind comprising at least one substrate on which at leastone display electrode is arranged which may be capacitively,galvanically or inductively coupled to an external object.

FIG. 1 a illustrates a top and side view of a portion of a display 10known per se. The leftmost figure in FIG. 1 a illustrates the well knownseven segment 11 arrangement, wherein different digits may be presenteddepending on which segments 11 that are active. Each segment 11 isreachable by means of thin wires 12 extending from the segment 11towards electrical terminals 13 normally provided on the edge of thedisplay 10. The segments 11 are formed on the inside of a frontsubstrate 14 and a back substrate 15 of the display 10. In this contextit is emphasized that the substrates used in the display may be made ofglass or plastic, on which a suitable electrical material, such asIndium Tin Oxide (ITO), is deposited as to form the segments 11, or oneor more substrates in the display may be made of an electric material,such as aluminium and shaped as to provide the segments 11. In e.g. OLEDdisplays a rib structure is pre-formed on patterned ITO anode lines on aglass substrate. Organic materials and cathode metal are deposited onthe substrate, wherein the rib structure automatically produces an OLEDdisplay with electrical isolation for metallic cathode lines formed ontop of the deposited organic materials. Depending on the displaytechnique used the display may comprise further elements besides thefront substrate 14 and the back substrate 15, which elements are notshown for sake of clarity. For example the display may also comprise afirst polarizer arranged on top of the front substrate 14 and a secondpolarizer arranged below the back substrate 15. In addition to thepolarizers, the space between the front substrate 14 and the backsubstrate 15 may be filled with liquid crystals 16 in a manner known perse.

The rightmost figure in FIG. 1 a illustrates an alternative design ofthe display electrodes 11 on the display 10. Instead of the sevensegment 11 arrangement the display electrodes 11 are arranged as amatrix of pixels 11′. At the cost of more wires 12 and terminals 13,this arrangement facilitates the presentation of more complex figuresthan the seven segment 11 arrangement. The display functionality of thematrix arrangement of pixels is, however, the same as with the sevensegment 11 arrangement. In this context it is appreciated that the termsegment is used for describing a display electrode on a substrate or ina metallic layer in a display. The term shall not be interpreted as onlydescribing a display electrode in a seven-segment arrangement, but maybe an electrode of any shape, e.g. a pixel in a matrix arrangement asdisclosed above.

The segments 11 on the back substrate 15 are normally interconnected soas to minimize the amount of wires 12 and terminals 13 on the display,i.e. the segments 11 on the back substrate 15 will always have the samepotential, whereas shapes on the display 10 are presented by means ofchanging the potential of the segments 11 on the front substrate 14 inrelation to the potential on the segments 11 on the back substrate 15.

FIG. 1 b is a simplified view of the allocation of some of thestray-capacitances in an LCD display 10. The spacing of the substrates14, 15 in the figure is exaggerated for the sake of clarity. As can beseen in the figure a first capacitance C1 stretches from the segments 11on the front substrate 14 towards the segments 11 on the back substrate15. The major contribution to C1 is the capacitance between the segments11 on front and back substrates that are on top of each other. It is,however, appreciated that the capacitance C1 also includes the straycapacitances between each segment 11 on the front substrate 14 and allsegments 11 on the back substrate 15.

A second capacitance C2, C2′ appears between different segments 11 oneach substrate 14, 15. The major contribution to C2 is the capacitancebetween adjacent segments, but it is understood that C2 also includesthe capacitance between one specific segment 11 and all other segments11 on the same substrate 14, 15.

When a user of the touch sensor touches the display a third capacitanceC3, C3′ appears between the segments 11 on the front 14 and backsubstrate 15 and the finger 17 of the user. The value of the thirdcapacitance C3, C3′ depends inter alia on the thickness of thesubstrates and the properties of the object touching the display 10.

A fourth capacitance C4 stretches from each and every segment towardsground potential via the environment and depends on the distance to theclosest ground reference as well as on the properties of the environment(i.e. the dielectric constant of the air in the environment, therelative humidity, etc.).

As to the size of the different stray capacitances the value of C1 is byfar greater than C2 and C3 due to the close spacing between the frontsubstrate 14 and the back substrate 15. For the same reason the sizes ofC3 and C3′ are almost equal whereas the value of C2 depends on the sizedisplay 10 as well as on the spacing of the segments 11. In case thesegments 11 on the back substrate 15 are interconnected, the straycapacitance C2′ becomes negligible compared to the galvanic contactprovided by the thin interconnecting wires 12 on the substrate 15. Anincrease in the capacitance C2 due to a touch on the display coveringtwo adjacent segments will hence be hard to detect due to the relativelylarge capacitance C1 and the short-circuited segments on the backsubstrate.

FIG. 2 illustrates a first embodiment of a touch sensor 20 according tothe present invention. An interface 21 is coupled to the display drivercircuitry (not shown). It is emphasized that the display drivercircuitry is not especially adapted for the touch display according tothe present invention, but may on the contrary be manufactured fordriving ordinary displays without touch sensitivity. The interface mayin its simplest form be a contact providing the display driver circuitrywith electric connections to the display electrodes 11 on the display10. Alternatively the interface comprises buffers and impedance matchingmeans for providing the display driver circuitry with an optimumoperating point thereby increasing the working time of the displaydriver.

The interface 21 is coupled to a set of switches 22 which in a firststate of operation connects the interface 21 to the display electrodes11 on the front substrate 14 and the back substrate 15 of the display10. In FIG. 2 only the switches 22 associated with one pair of segments11 are illustrated for the sake of clarity; however, the dashed lines inthe figure indicates that each segment 11 or group of segments 11 incase the segments 11 on the back substrate 15 are interconnected(partially or completely) on the front substrate 14 and the backsubstrate 15 are connected to the interface 21 by means of a switch 22.In a preferred embodiment of the present invention, the segments 11 onthe back substrate are not interconnected but are individually reachablewithin the touch sensitive device 20. The interface 21 groups the wires23 from the segments 11 on the back substrate 15 making it possible touse standard display driver circuitry adapted for driving displays witha common electrode on the back substrate 15. As will be disclosed belowthe accuracy of the touch sensor is improved by not interconnecting thewires 23 from the segments 11 on the back plane 15 until they reach theinterface 21, thereby making it possible to isolate each segment 11 bymeans of the switches 22. By not interconnecting the segments 11 on theback segment 15 it is also possible to detect two or more touches on thedisplay 10 simultaneously, i.e. it is possible to distinguish a touch bya finger from an unintentional touch by the whole hand normally referredto as “palm rejection”.

When the switches 22 are in the first state of operation the display 10is not sensitive to touches on the surface thereof, but acts as anordinary display. However, a control unit 24 in the touch sensor 20operates the switches 21 in the device so as to put them in a secondstate of operation, wherein the display 10 is disconnected from theinterface 21. Instead the segments 11 on one of the substrates areconnected to a signal generator 25 which feeds a test signal to thesegments 11. In the figure the segments 11 on the front substrate 14 areconnected to the signal generator 25, but in an alternative embodimentthe segments 11 on the back substrate 15 rather than the segments 11 onthe front substrate 14 may be connected to the signal generator 25. Asdisclosed above, the relatively large capacitance C1 makes it equallypossible to connect either the segments 11 on the front substrate 14 orthe segments 11 on the back substrate 15 to the signal generator 25without loosing functionality of the touch sensitive device.

As the segments 11 on the front substrate 14 are connected to the signalgenerator 25, the segments 11 on the back substrate 15 are left in ahigh-impedance state either by simply disconnecting them from theinterface 21 or, as shown in FIG. 2, connect them to signal ground via ahigh-ohmic resistor 26. As disclosed above, the information on thedisplay 10 depends on the difference in potential between the segments11 on the front substrate 14 and the segments 11 on the back substrate15. The high-ohmic state of the segments 11 on the back substrate 15 andthe relatively large capacitance C1 will ensure that any difference inpotential between different segments 11 on the substrates 14, 15 ispreserved even though a test signal is applied to the segments 11 on oneof the substrates 14, 15. A change in potential on a segment on thefront substrate 14 will hence change the potential on the segment 11arranged directly below on the back substrate 15. Consequently, theinformation presented on the display when the switches 22 are in thefirst state of operation will be preserved when the switches 22 connectsthe signal generator 25 to the segments 11 on the front substrate 14 inthe second state of operation.

A signal evaluation circuit 27 is coupled to the segments 11 on thefront substrate 14. Since the capacitances C1 and C2 of the display arewell known and are established when the display is manufactured, theresponse to the test signal by the display when no foreign objecttouches the display is also well known. When the operator of the deviceputs his finger on the display, the capacitance C3 disclosed above willbecome part of the load presented to the signal generator. The responseto the test signal will hence be changed indicating to the signalevaluation circuitry 27 the presence of a touch on one or more of thesegments 11 on the display 10. Since all segments 11 on the frontsubstrate 14 are connected to the signal evaluation circuitry 27 it maydetermine which segment 11 or segments 11 that are affected by thetouch. The signal evaluation circuitry may then respond to the touch byeither providing a general “key-down”-signal or preferably more detailedinformation regarding which specific segments 11 that are affected bythe touch to an external control unit (not shown).

FIG. 3 illustrates the function of the signal generator 25 and thesignal evaluation circuitry 27 according to a first embodiment of thepresent invention. When the switches are in the second state ofoperation, the signal generator 25 feeds a square wave 31 to thesegments 11 on the front substrate 14 via a set of resistors 32. In thefigure only two resistors 32 are shown for the sake of clarity. Theactual number of resistors 32 depends on the number of segments 11 thatare to be used for detecting a touch on the display. Since the segments11 on the back substrate 15 are disconnected and left in a high-ohmicstate the load presented by the segments 11 alone will become thecapacitances C1 and C2 in series with the small capacitance C4 in FIG. 1b. Hence by leaving the segments 11 on the back plane 15 in a high-ohmicstate the large capacitor C1 will become series-coupled with the smallcapacitance C4 making the contribution of C1 less dominant. In case theelectrodes 11 on the back substrate 15 are interconnected the accuracyof the touch sensor will be somewhat deteriorated due to thecapacity-coupling between different segments 11 on the front substrate14 via the short-circuited back segments 11 and the capacitances C1between each front and back segment 11. The small capacitance presentedby the coupling of C1, C2 and C4 will slightly change the appearance ofthe test signal 33 at a point to the right of the resistors in FIG. 3.Instead of a square wave, the test signal exhibits the well knowexponential increase in potential due to the charging of capacitancesC1, C2, and C4 via the fixed resistors 32. Preferably the rise or falltime of the loaded test signals are measured by the signal evaluationcircuitry 27 so as to determine if the capacitive load has changed.Small variations in the rise time may occur due to changes in theenvironment in which the touch sensor 20 is operating. These smallchanges will not give rise to an output signal from the signalevaluation circuitry 27 indicating a touch on the display 10, but areaccepted as environment-induced variations in the test signal.

When the operator 17 of the device 20 touches the display 10, thecapacitive load presented to the signal generator 25 will increase dueto the capacitance C3 hence increasing the rise and fall time of thetest signal 34 to the right of the resistors 32. The capacitance C3 islarge compared to the series connection C1, C2, and C4 thereby making agreat contribution to the overall capacitance presented to the signalgenerator. The exact magnitude of the increase in the rise time is notcritical as long as it is large enough for making it possible todistinguish a touch on the display from the small environment-inducedvariations disclosed above. The signal evaluation circuitry may be inform of a simple comparator providing an output signal in case the risetime exceeds a predetermined value, or may be intelligent in that itanalyses the long time behaviour of the rise time and compensates forchanges in the environment.

The control unit 24 is adapted to alternate the switches 22 between thefirst and second state of operation. The rate at which the control unit24 change the state of the switches 22 depends on the capacitance C1,the resistance between each segment and signal ground, and the inertiain the liquid crystal, i.e. how long it takes for the crystals in thedisplay to turn in the absence of an external electric field.

The resistances 32 in FIG. 3 may be implemented in form of traditionalresistors 32 or as shown in an alternative embodiment in FIG. 4 asswitched capacitors 42. The switched capacitor 42 known per se is wellsuited for integration on a chip making it possible to combine theelectronics of the touch sensor with the display 10 as an integral unit.

In the above embodiment of the present invention a capacitance measuringcircuit in form of a signal generator 25 and a signal evaluation circuit27 is disclosed, wherein the signal evaluation circuit 27 measures therise or fall time of the test signal. It is however appreciated that thecapacitance measuring circuit as well may measure the current fed to thedisplay at a fixed or varied voltage and frequency, measure the phasedifference between current and voltage applied to the display, ormeasure the capacitance between at least one segment 11 of the display10 and the environment in any other suitable way.

In an alternative embodiment of the present invention the displayelectrode 11 may be arranged on a substrate 14, 15 so as to make itpossible to detect a galvanic contact between the display electrode 11and an object touching the display device 10. For example, a display maybe formed by a matrix of light emitting diodes (LED), wherein each diodeof the display is soldered to a pair of pads on a printed circuit board(PCB). Each pad then constitutes a display electrode 11 which may bedisconnected from the display driver circuitry and used for detecting atouch on the display device 10. Hence, if a person touches the displayelectrode 11, the person will act as a capacitor receiving charge fromthe display electrodes 11. A small, detectable current will flow fromthe display device 10 through the finger of the person indicating atouch on the display device 10.

In yet an alternative embodiment of the present invention the displayelectrode 11 may be provided with a high-voltage when the switches arein the second state of operation. The display electrodes 11 on thesubstrates 14, 15 may then be arranged between the substrates 14, 15, asdisclosed with regards to the LCD display above, wherein a very largeresistance of the substrate still may allow the flow of a current largeenough to be detected when a person touches the front of the displaydevice 10.

In yet an alternative embodiment of the invention, touch sensitive areasare formed on one side of a third substrate in accordance with thedescription in relation to FIG. 1 (i.e. a substrate e.g. made of glassor plastic, on which a suitable electrical material, such as Indium TinOxide (ITO), is deposited as to form the desired touch-sensitive areas).The third substrate may then be arranged in front of any kind of displayin order to provide touch-sensitivity for the display. Thetouch-sensitive areas are preferably arranged on the inside of thesubstrate, i.e. the side which is facing the display and is not indirect contact with the users of the touch-sensitive substrate, in orderto provide a long operational life of the touch-sensitive substrate evenunder exposure to hard wear. The third substrate will, in addition toprovide touch sensitivity for the display, also act as a protectivecover since it is arranged in front of the display.

1. A touch sensor, comprising: a display device having a first substrateon which at least one display electrode is disposed, and a secondsubstrate on which at least one display electrode is disposed, for thedisplay of a shape on the display device; an interface coupled to the atleast one display electrode on the first substrate for receiving displaydata to the display device, the display data providing a voltage valuebetween the at least one display electrode on the first substrate andthe at least one display electrode on the second substrate; a measuringcircuit coupled to the at least one display electrode on the firstsubstrate; and switching means for connecting the interface to the atleast one display electrodes on the first and second substrates when theswitching means is in a first state of operation and for connecting themeasuring circuit to the at least one display electrode on the firstsubstrate when the switching means is in a second state of operation,wherein: the measuring circuit is configured to detect a change incapacitance or resistance measured at the at least one display electrodeon the first substrate; and the switching means is configured topreserve the voltage value between the at least one display electrode onthe first substrate and the at least one display electrode on the secondsubstrate when the switching means is in the second state of operation.2. A touch sensor according to claim 1, wherein the measuring circuitcomprises: a signal generator coupled to the at least one displayelectrode for providing a predetermined test signal to the displayelectrode; and a signal evaluating circuit coupled to the at least onedisplay electrode for receiving the test signal from the signalgenerator.
 3. A touch sensor according to claim 2, wherein the signalevaluation circuit is configured to detect a deviation in the testsignal when the switching means is in the second state of operation. 4.A touch sensor according to claim 2, wherein: the display devicecomprises a front substrate having a plurality of segments; and thesignal generator is configured to apply the test signal to the segmentson the front substrate.
 5. A touch sensor according to claim 2, wherein:the display device comprises a back substrate having a plurality ofsegments; and the signal generator is configured to apply the testsignal to segments on the back substrate.
 6. A touch sensor according toclaim 4, wherein the segments on the front substrate which are notconnected to the signal generator are retained in a high-ohmic state. 7.A method for detecting a touch on a display device, said display devicehaving a first substrate on which at least one display electrode isdisposed and a second substrate on which at least one display electrodeis disposed, for the display of a shape on the display device, whereinsaid display electrodes are coupled to an interface for receivingdisplay data to the display device, comprising the steps of:disconnecting the at least one display electrode on the first and secondsubstrate from the interface; connecting said display electrode on thefirst substrate to a measuring circuit; and detecting, by the measuringcircuit, a change in a capacitance or resistance of the at least onedisplay electrode on the first substrate due to an electrical couplingwith an object touching the display device in the vicinity of thedisplay electrode on the first substrate, wherein the at least onedisplay electrode on the second substrate is connected such that avoltage level between the at least one display electrode on the firstsubstrate and the at least one display electrode on the second substrateis preserved.
 8. A method according to claim 7, wherein detecting achange in a capacitance or resistance of the display electrodecomprises: applying a predetermined test signal to the display electrodeand detecting a deviation in the test signal due to an electricalcoupling with an object touching the display device in the vicinity ofthe display electrode.
 9. A method according to claim 7, wherein theelectrical coupling comprises a capacitive coupling.
 10. A methodaccording to claim 7, wherein the electrical coupling comprises agalvanic coupling.
 11. A method according to claim 8, wherein theelectrical coupling comprises a capacitive coupling.
 12. A methodaccording to claim 8, wherein the electrical coupling comprises agalvanic coupling.
 13. A touch sensor according to claim 3, wherein: thedisplay device comprises a front substrate having a plurality ofsegments; and the signal generator is adapted to apply the test signalto the segments on the front substrate.
 14. A touch sensor according toclaim 3, wherein: the display device comprises a back substrate having aplurality of segments; and the signal generator is adapted to apply thetest signal to the segments on the back substrate.
 15. A touch sensoraccording to claim 5, wherein the segments on the back substrate whichare not connected to the signal generator are retained in a high-ohmicstate.